Optimization and analysis of deposition processes of amorphous silicon for silicon heterojunction solar cells
Intrinsic hydrogenated amorphous silicon a-Si:H(i) offers a very good passivation quality. Adding dopants to the deposition reduces the passivation quality. However, sufficient doping is needed to build the p-n-junction which is essential to gain an actual voltage. Furthermore, the conductivity of the a-Si:H is also a function of the dopant concentration. Poor conductivities in the emitter layer causes high series resistances, limiting fill factor and cell efficiency. To achieve good passivation quality, sufficient band bending and low resistivity simultaneously, we have introduced a new approach of applying a doping profile to the deposition of the amorphous emitter layer. The conventional concept to process silicon heterojunction (SHJ) solar cells is to deposit a heavily doped a:Si-H(n or p) layer on a thin intrinsic one. In this paper we will compare structures with gradually doped emitter profiles to conventional ones. Gradually doped emitters are processed in one single deposition step, which offers simple and fast processing. The total electrical conductivity can be increased by a well chosen doping profile. This is shown by simulations based on conductivity measurements and full heterojunction solar cells. The best n-type, FZ wafer heterojunction solar cell realized within this first investigation has an efficiency of 18.9%.